We must ‘undo’ these operations, in reverse order,
to get back to
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$\,x\,$.
Note that addition is undone with subtraction, and multiplication is undone with division:[beautiful math coming... please be patient]
$$
ax + b \qquad\overset{\text{subtract $b$}}{\rightarrow}\qquad
ax \qquad\overset{\text{divide by $a$}}{\rightarrow}\qquad
x
$$

Thus, we start with the original equation, first subtract
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$\,b\,$ from both sides, and then divide both sides by
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$\,a\,$: